Can I create a separate Volumetric pass for Blender Cycles 2.70?
There is not a Volume Pass, but you can make one by subtracting everything else out of the beauty pass.

From http://blenderartists.org/forum/showthread.php?380260-Cycles-Volume-Pass


  Since there is no default Volume Pass checkbox in Cycles yet I put together a compositor node group which just simply extracts it out of the beauty pass by subtracting diffuse, glossy and transmission component. To handle volume pass separately, blur it for example and than add it back without a noise can be rather time-saving. 


There's also a node group you can download from that thread and examine.